Differential effects of caspase inhibitors on TNF-induced necroptosis.

TNF has been reported to induce caspase-independent necroptosis in the presence of Z-VAD-fmk, a pan-caspase inhibitor. We examined whether necroptosis was induced by caspase inhibitors other than Z-VAD-fmk. TNF-induced necroptosis was detected in the presence of Z-DEVD-fmk, which is commonly used as a caspase-3-specific inhibitor, but not in the presence of Z-Asp-CH2-DCB, which was reported to be a pan-caspase inhibitor. TNF-induced caspase-3 activity was completely inhibited by Z-VAD-fmk, Z-DEVD-fmk, or Z-Asp-CH2-DCB. Although TNF-induced proteolytic activation of procaspase-3 was completely prevented by Z-VAD-fmk or Z-DEVD-fmk, the partial proteolysis of procaspase-3 was induced in the presence of Z-Asp-CH2-DCB. Furthermore, although TNF-induced proteolytic activation of procaspase-8 was completely inhibited by Z-VAD-fmk or Z-DEVD-fmk, the partial proteolysis of procaspase-8 to the p43/41 intermediate and p18 active fragment was detected in the presence of Z-Asp-CH2-DCB. The cleavage of RIP1, which plays a crucial role in TNF-induced necroptosis and is cleaved by caspase-8, was completely inhibited by Z-VAD-fmk or Z-DEVD-fmk, whereas the partial degradation of RIP1 was detected in the presence of Z-Asp-CH2-DCB. These results suggest that the partial activation of caspase-8 in the presence of Z-Asp-CH2-DCB may suppress TNF-induced necroptosis via the cleavage of RIP1, and also suggest that Z-Asp-CH2-DCB, but not Z-DEVD-fmk, may be used as a caspase-3-specific inhibitor in cells.